Although, due to improvements in materials science, modern composite fillers are a permanent fixture in the treatments available to dentists, even in the side tooth area, these systems nevertheless have several fundamental drawbacks which are primarily linked with the ‘bond’ between the organic resin matrix and the inorganic filler surfaces. The silane coupling agents form ‘siloxane bonds’ with minerals. These bonds which ensure the bond between the two phases, may be hydrolysed, like any bond between an organic polymer and a hydrophilic, mineral material surface. Hydrolysis of the siloxane bond however produces hydrolytic degradation in the polymer, increased crack formation along the material/resin interfacial region, water absorption, softening effects in the polymer, swelling of the composite, reduced wear resistance, abrasion resistance and colour stability, due to the filler breaking out. Finally, the bond of the two phases is broken.
The advantage of silane relative to other adhesion promoters lies in its characteristic of behaving in a reversible manner with regard to hydrolytic bond cleavage. The thermodynamic equilibrium lies broadly on the side of the siloxane bond formation. Although the equilibrium amount of water molecules is therefore more important on the polymer/solid interface layer than the diffusion rate of the water in the polymer, water entering the material will however set the hydrolytic degradation process in motion. In the presence of strongly hydrophobic resins, water itself reaches the polymer/solid interface by diffusion. Once the interface layer is attacked, the water is attached there in the form of clusters, the bond of the organic phase to the inorganic phase is loosened and the structure of the composite broken up by osmotic pressure.
To improve the bond between filler and polymer matrix, the possibility was considered to create a physical adhesion in addition to the chemical adhesion. In U.S. Pat. No. 4,215,033 a semi-porous filler is produced by etching glass. Microporous fillers for use as dental materials are known from the publications U.S. Pat. No. 4,217,264, EP 4868, EP 172513, DE 19846556 and DE 19615763. With the physical adhesion, resin penetrates into the pores of the filler and thus after polymerisation anchors the organic with the inorganic phase, as the cured resin is held tightly in the pores of the filler. Thus an improved structural integrity of the moulded material is ensured.
The principle of the physical anchoring of filler and matrix which is disclosed in DE 19615763 and includes the use of porous SiO2 particles, has however three principal disadvantages. The first consists of the extremely expensive production of the porous filling materials which includes a very expensive phase separation step, and grinding and screening processes. The second disadvantage lies in the very small pore diameter which is preferably 90-100 nanometers. In order to ensure an effective inflow of the resin into the pores, resin composites of very low viscosity must be used with low surface tension. This is obtained by the use of dimethacrylates with a low molecular weight, such as for example triethyleneglycoldimethacrylate (TEDMA) or hexanedioldimethacrylate (HDDMA). A higher proportion of these low molecular monomers leads however to increased composite shrinkage. Alternatively, the viscosity of the matrix can also be reduced by the addition of monomethacrylates, such as for example hydroxypropylmethacrylate (HPMA) or triethyleneglycolmonoethylethermonomethacrylate. The use of monomethacrylates leads to a poorer cross-linking of the polymer, compared to dimethacrylates and thus to lower flexural strength and greater discolouration. The third disadvantage lies in the restriction of the production process to silicon dioxide fillers which do not allow a clinically acceptable radiopacity to be set.
In spite of enormous improvements in the field of dental composite materials, the problem of the phase bond however remains unsolved, as even with the use of porous fillers, it can lead to the release of the polymer matrix from the inorganic filler by means of hydrolytic cleavage. It is therefore the object of the invention to provide a filler which forms a stable bond with the organic phase and allows such a strong physical bond between it and the binder of the dental material that possible hydrolysis can no longer destroy the bond once it is formed, and a composite material containing this filler which, due to the stable bond between the phases, ensures improved properties relative to the prior art.